This invention relates to a triangulation range camera and more particularly to a high speed, sensitive wide range coded aperture light detector for these and other systems
Devices such as a tree dimensional camera based on triangulation may require measurement speeds up to 10 million range elements (rangels) per second In this application the range measurement is obtained from the position of a spot of bright illumination, often in a varied diffuse background. Spot resolution to at least 8 bits (256 point resolution) typically may be required. Conventional approaches to this problem are summarized below and are too slow or of limited sensitivity, because of detector device limitations and because each range point must be processed sequentially A faster range readout method is needed which preserves the signal verification capabilities of known approaches.
Conventional triangulation range cameras use linear detector arrays, two dimensional arrays, or flying spot configurations. The position of a light spot on a linear array can be determined by reading out each pixel of the array and locating the pixel with the strongest recorded signal, or the center of a peak covering several pixels. If there are 256 pixels and the pixel readout rate is a typical 10 MHz, then 25.6 microseconds is required for a range measurement to a single point and the range measurement rate is 40 kHz, which is a factor of 100 below typical desired rates. A two-dimensional array such as used in CID (charge injection device) and CCD (charge coupled device) TV cameras allows multiple range points to be determined, one from each line of the array, if the light source is a line or a point swept over a line during the array exposure. However the pixel readout rate from a conventional 2-D array is still limited to approximately 10 MHz, providing no advantage in range measurement rate since over 100 pixels must be read to provide a typical range measurement. Furthermore, present solid state cameras are much less sensitive to light, and have much smaller dynamic range than the best available light detectors, e.g. photomultipliers. Since those qualities are also important for robust high speed measurements, solid state detectors should either be improved or replaced for such applications.
The advantages of photomultipliers detectors have been employed in flying spot triangulation rangers, wherein range is determined point by point from the timing of a pulse. One example is U.S. Pat. No. 4,645,917 to Penney, Roy and Thomas. A disadvantage of this approach is that it is slow, because one must wait for the pulse. Range can also be determined by measuring the light spot position using a linear position sensing diode or a photomultiplier pair viewing the spot through density wedges. However, the former is intrinsically slow, limited by detector properties to range data rates much less than 1 MHz for low light level signals. The photomultiplier-density wedge combination can provide range data rates on the order of 10 MHz for the same light levels; it senses the position of the centroid of light illumination rather than the position of the sharp point of light carrying range information. Thus the photomultipliers pair can give substantially incorrect range information whenever there is significant background illumination or secondary scattering from the projected beam. Furthermore, the signals from the density wedge detectors do not provide any signal quality information, which is necessary for good range data from shiny targets. Finally, a signal to noise analysis shows that analog light spot position measuring devices are less efficient than optimum digital equivalents when resolutions better than one part in 64 are required, as in the present case.